Protectors for electric circuits



Dec. 17, 1968 J. s. WITHERS PROTECTORS FOR ELECTRIC CIRCUITS Filed Dec. 22, 1966 r 0 w 7 in a u n ,0 n 2 n n n n n n. 2 m

FIG.3.

United States Patent Oflice 3,417,357 PROTECTORS FOR ELECTRIC CIRCUITS John S. Withers, Dellwood, Mo., assignor to McGraw- Edison Company, Elgin, 11]., a corporation of Delaware Filed Dec. 22, 1966, Ser. No. 604,015 14 Claims. (Cl. 337-295) ABSTRACT OF THE DISCLOSURE The weak spots in the fusible elements of electric fuses are formed so the effective lengths thereof are zeroto minimize the electrical resistances of those fusible elements and to keep those weak spots relatively cool which those electric fuses carry their rated currents.

This invention relates to improvements in protectors for electric circuits. More particularly, this invention relates to improvements in electric fuses.

It is, therefore, an object of the present invention to provide an improved electric fuse.

The fusible elements of many electric fuses have portions of reduced cross section that are known as weak spots. Customarily, those weak spots are formed by punching notches in the opposite edges of those fusible elements to define centrally-located portions of reduced cross section or by punching openings in those fusible elements to define spaced portions of reduced cross section. The weak spots of fusible elements account for the bulk of the electrical resistances of those fusible elements; and, to minimize the electrical resistances of those fusible elements, it would be desirable to have the weak spots of those fusible elements as short as possible. The present invention provides weak spots, for fusible elements, that effectively have zero lengths, and thus enables those fusible elements to have low resistances. It is, therefore, an object of the present invention to provide a weak spot, for a fusible element of an electric fuse, which effectively has zero length.

The present invention provides a weak spot that effectively has zero length by using adjacent but axiallyotfset open areas in a fusible element to define that weak spot. Where one corner of one of those open areas is in register with one corner of the other of those open areas, and where the rest of those open areas are axially spaced away from each other, the portion of the fusible element between those corners is essentially a zero length weak spot. It is, therefore, an object of the present invention to provide a fusible element with adjacent but axially-offset open areas therein, with one corner of one of those open areas in register with one corner of the other of those open areas, and with the rest of those open areas axially spaced away from each other.

In a fusible element which has one corner of an open area in register with one corner of an axially-displaced open area to define an effectively zero length weak spot, the cross sections of those portions of that fusible element which are on opposite sides of that weak spot will be greater than the cross section at that weak spot. The greater cross sections of those portions of the fusible element which are on opposite sides of the effectively zero length weak spot will enable those portions to have resistances which are much smaller than the resistance of that weak spot. Also, the greater cross sections of those portions of the fusible element which are on oppo- 3,417,357 Patented Dec. 17, 1968 site sides of the effectively zero length weak spot will enable those portions to rapidly conduct away the heat generated by current flowing through that weak spot. As a result, the resistance of the weak spot will effectively account for the resistance of the fusible element, and that weak spot can respond to the rated current of that fusible element to generate far more heat per unit of length than can a standard weak spot of equal width and resistivity.

Othef and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying descripition four preferred embodiments of the present invention are shown and described but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

In the drawing, FIG. 1 is a longitudinal section through one preferred embodiment of electric fuse that is made in accordance with the principles and teachings of the present invention,

FIG. 2 is a plan view, on a larger scale, of the central portion of the fusible element of the electric fuse shown in FIG. 1,

FIG. 3 is a plan view, on the scale of FIG. 2, of the central portion of another fusible element which could be used in the electric fuse of FIG. 1,

FIG. 4 is a plan view, on the scale of FIG. 2, of the central portion of yet another fusible element which could be used in the electric fuse of FIG. 1, and

FIG. 5 is a plan view, on the scale of FIG. 2, of the central portion of a further fusible element which could be used in the electric fuse of FIG. 1.

Referring to the drawing in detail, the numeral 10 generally denotes one preferred embodiment of electric fuse that is made in accordance with the principles and teachings of the present invention. That electric fuse has a tubular casing 12 of insulating material; and that insulating material can be fibre, glass, glass melamine, ceramic, or any of the other insulating materials customarily used in the art of electric fuses. Ferrule-like metal terminals 14 and 16 are telescoped over the opposite ends of the casing 12; and masses 18 and 20 of solder secure a fusible element 22 to those ferrule-like terminals.

The fusible element 22 preferably is an elongated strip of metal; and it has a notch 24 in one edge thereof and has a second notch 26 in the opposite edge thereof. The notches 24 and 26 are rectangular in configuration, and those notches are displaced axially of the fusible element 22; but the inner corner of the notch 24 which is denoted by the numeral 28 is directly in register with the inner corner of the notch 26 which is denoted by the numeral 30. The portion 31 of the fusible element 22 which is located directly between the inner corners 28 and 30, respectively, of the notches 24 and 26, constitutes the weak spot for the fusible element 22.

The portions of the fusible element 22 which are located to the left of the weak spot 31 have cross sections that are very much greater than the cross-section of that weak spot. Similarly, the portions of the fusible element 22 which are located to the right of the Weak spot 31 have cross sections that are very much greater than the cross section of that weak spot. As a result, the weak spot 31 effectively is a zero length weak spot.

The very much greater cross sections of the portions of the fusible element 22 which are located to the left and to the right of the weak spot 31 make the electrical resistances of those portions very much less than the electrical resistance of that weak spot. Also, the very much larger cross sections of those portions enable those portions to rapidly conduct heat, generated at the weak spot 31, away from that weak spot. All of this is important; because it keeps the total resistance of the fusible element 22 unusually low, and because it enables the weak spot 31 to remain relatively cool while the electric fuse is carrying its rated current, although the effective length of that weak spot is effectively zero. The zero length weak spot 31 is additionally important because it will generate less peak recovery voltage when it melts than would a corresponding width weak spot formed by punching two V-shaped notches in register with each other or by punching an opening in the center of a fusible element. Furthermore, the zero length weak spot 31 will melt out faster on a heavy short circuit than would a corresponding width weak spot formed by two V- shaped notches in register with each other or by an opening in the center of a fusible element. As a result, the electric fuse 10 will be able to carry its rated current indefinitely, but will be able to respond to potentially hurtful overloads to quickly fuse and thereby safely open the circuit.

The axial displacement of the notches 24 and 26 of the fusible element 22 is additionally helpful, because it permits that fusible element to be formed by a die which is less expensive to machine and by a punch which requires less maintenance than do the dies and punches used to form conventional weak spots in fusible elements. Thus, where a weak spot in a fusible element is defined by two V-shaped notches that are in the opposite edges of a fusible element and that are in register with each other, the acute angle die that helps form those notches must be machined with an extremely high degree of accuracy; whereas the degree of accuracy that is commonly used in forming dies is all that is needed to form the large angle-rectangular-die that will be used in forming the notches 24 and 26 of FIGS. 1 and 2. Also, where a weak spot in a fusible element is defined by two V-shaped notches that are in the opposite edges of a fusible element and that are in register wtih each other, the acute angle punch that helps form those notches will wear far more rapidly, and will thus require for more maintenance, than will the large angle-rectangularpunch that will be used in forming the notches 24 and 26 of FIGS. 1 and 2. Moreover, because the large angle punch that will be used in forming the notches 24 and 26 of FIGS. 1 and 2 will experience less wear than would an acute angle punch used to form two V-shaped notches, the width of the weak spot 31 will be far less subject to variations, on a production line basis of manufacture, than would the width of a weak spot defined by the two V-shaped notches.

The electric fuse 10 of FIG. 1 is shown as having just one fusible element 22; and that fusible element is shown as having just one weak spot 31. However, electric fuses made in accordance with the principles and teachings of the present invention can have any desired number of fusible elements; and each of those fusible elements I can have any desired number of weak spots formed along the lengths thereof. In the fusible element 22 of FIG. 1, the notch 24 coacts with the notch 26 to form just one Zero length weak spot. If a further notch 26 were to be formed and were to have the right-hand edge thereof in alignment with the left-hand edge of the notch 24, the notch 24 would coact with that further notch 26 and with the illustrated notch 26 to define two axiallyspaced zero length weak, spots.

Referring particularly to FIG. 3, the numeral 32 generally denotes a fusible element which could be substituted for the fusible element 22 in the fuse 10 of FIG. 1. The

fusible element 32 has two notches 34 and 36 which are displaced axially of that fusible element. Each of those notches has the configuration of a truncated triangle; and the inner corner 38 of the notch 34 is in register with the inner corner 40 of the notch 36. The portion of the fusible element 32 which is located directly between the inner corners 38 and 40, respectively, of the notches 34 and 36 constitutes a zero length weak spot 41. The portion of the fusible element 32 which is located to the left of that weak spot has a progressively larger width than does that weak spot; and the portion of that fusible element which is located to the right of that Weak spot has a progressively larger width does that weak spot. The widths of the portions which are located to the left and to the right of the weak spot 41 of the fusible element 32 do not increase as sharply as do the widths of the portions which are located to the left and to the right of the weak spot 31 of the fusible element 22. However, the widths of the portions which are located to the left and to the right of the weak spot 41 of the fusible element 32 increase sufificiently sharply to enable those portions of that fusible element to keep the electrical resistance of that fusible element low and to enable those portions to quickly absorb most of the heat generated at that weak spot. The fusible element 32 of FIG. 3 will possess most of the advantages of the fusible element 22 of FIG. 1, but it will have a slightly greater electrical resistance, and it will operate at a slightly higher temperature. The die and punch used to form the notches 34 and 36 of FIG. 3 will be a largeobtuse-angle die and a largeobtuseangle punch; and hence that die and punch will be cheaper to machine and will be easier to maintain than would be acute angle die and the acute angle punch needed to form two V-shaped notches in the opposite edges of a fusible element.

Referriag particularly to FIG. 4, the numeral 42 generally denotes a fusible element which could be substituted for the fusible element 22 in the fuse 10 of FIG. 1. The fusible element 42 has two openings 44 and 46 which are displaced axially of that fusible element. Each of those openings is rectangular in configuration; and the inner corner 48 of the opening 44 is in register with the inner corner 50 of the opening 46. The portion of the fusible element 42 which is located directly between the inner corners 48 and 50, respectively, of the openings 44 and 46 constitutes a zero length weak spot 52. The portion of the fusible element 42 which is located to the left of that Weak spot has a very much greater width than does that weak spot; and the portion of that fusible element which is located to the right of that Weak spot has a very much greater width than does that weak spot. The widths of the portions which are located to the left and to-the right of the weak spot 52 of the fusible element 42 increase as sharply as do the Widths of the portions which are located to the left and to the right of the weak spot 31 of the fusible element 22. Hence, the widths of the portions which are located to the left and to the right of the weak spot 52 of the fusible element 42 increase sufiiciently sharply to enable those portions of that fusible element to keep the electrical resistance of that fusible element low and to enable those portions to quickly absorb most of the heat generated at that weak spot. The fusible element 42 of FIG. 4 will possess most of the advantages of the fusible element 22 of FIG. 1, but it will have Weak spots 54 and 56 in addition to the weak spot 52. Those Weak spots will effectively be in parallel with the weak spot 52; and they will cause the fusible element 42 to have an electrical resistance which is slightly greater than that of the fusible element 22, and they will cause that fusible element to operate at a temperature which is slightly higher than that of the fusible element 22.

Referring particularly to FIG. 5, the numeral 62 generally denotes a fusible element which could be substituted for the fusible element 22 in the fuse 10 of FIG. 1. The

fusible element 62 has two notches 64 and 66 that are in register with each other and has an opening 68 that is displaced axially from those notches. Those notches and that opening are rectangular in configuration; and a corner 72 of that opening is in register with the inner corner 70 of the notch 64 while a corner 78 of that opening is in register with the inner corner 76'of the notch 66. The portion of the fusible element 62 which is located directly between the corners 70 and 72 constitutes a zero length weak spot 74; and the portion of that fusible element which is located directly between the corners 76 and 78 constitutes a zero length weak spot 80.

The portion of the fusible element 62 which is located to the left of the weak spot 74 has a very much larger width than does that weak spot; and the portion of that fusible element which is located to the right of that weak spot has a very much larger width than does that weak spot. The widths of the portions which are located to the left and to the right of the weak spot 74 of the fusible element 62 increase as sharply as do the widths of the portions which are located to the left and to the right of the weak spot 31 of the fusible element 22. Hence, the widths of the portions which are located to the left and to the right of the weak spot 74 of the fusible element 62 increase sufliciently sharply to enable those portions of that fusible element to keep the electrical resistance of that fusible element low and to enable those portions to quickly absorb most of the heat generated at that weak spot. Similarly, the portion of the fusible element 62 which is located to the left of the weak spot 80 has a very much larger width than does that weak spot; and the portion of that fusible element which is located to the right of that weak spot has a very much larger width than does that weak spot. The widths of the portions which are located to the left and to the right of the weak spot 80 of the fusible element 62 increase as sharply as do the widths of the portions which are located to the left and to the right of the weak spot 31 of the fusible element 22. Hence, the widths of the portions which are located to the left and to the right of the weak spot 80 of the fusible element 62 increase sufficiently sharply to enable those portions of that fusible element to keep the electrical resistance of that fusible element low and to enable those portions to quickly absorb most of the heat generated at that weak spot.

The dies and punches that will be used to form the notches and openings 64, 66, 44, 46 and 68 of FIGS. 4 and 5 will be large anglerectangular-dies and punches. Hence, those dies and punches will be cheaper to machine and will be easier to maintain than would the acute angle die and the acute angle punch needed to form two V- shaped notches in the opposite edges of a fusible element.

The Weak spots 31, 41, 52, 74 and 80 of FIGS. 15 are effectively Zero length weak spots and thus are very desirable. If fusible elements with slightly higher electrical resistances were acceptable, the notch 26 of FIGS. 1 and 2 could be shifted to the left or right, the notch 36 of FIG. 3 could be shifted to the left or right, the notch 46 of FIG. 4 could be shifted to the left or right, and the opening 68 of FIG. 5 could be shifted to the left or right. Shifting of any of those open spaces to the left or right would increase the length of the weak spot formed by that open space and the adjacent open space, and would thus increase the electrical resistance of that fusible element; but, where the shift was limited to a few thousandths of an inch, the resulting weak spot would approximate many of the advantages of an effectively zero length weak spot.

Whereas the drawing and accompanying description have shown and described four preferred embodiments of the present invention it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I claim is:

1. A fusible element for an electric fuse which has a notch in one edge thereof and which has a second notch in the opposite edge thereof, the first said notch'having a part of the inner end thereof substantially in register with a part of the inner end of said second notch to define a weak spot, said first notch having substantially all of the length of said inner end thereof displaced axially in one direction from said part of said first notch, said second notch having substantially all of the length of said inner end thereof displaced axially in the opposite direction from said part of said second notch, whereby said weak spot has a short effective length.

2. A fusible element as claimed in claim 1 wherein said part of said inner end of said first said notch is an inner corner of said first said notch, wherein said part of said inner end of said second notch is an inner corner of said second notch, and wherein said effective length of said weak spot is substantially zero.

3. A fusible element as claimed in claim 1 wherein said first said notch has the configuration of a rectangle, and wherein said second notch has the configuration of a rectangle.

4. A fusible element as claimed in claim 1 wherein said first said notch has the configuration of a truncated triangle, and wherein said second notch has the configuration of a truncated triangle.

5. A fusible element as claimed in claim 1 wherein said inner end of said first said notch is substantially parallel to said inner end of said second notch.

6. A fusible element as claimed in claim 1 wherein the angles at the inner corners of said notches are large.

7. A fusible element as claimed in claim 1 wherein the angles at the inner corners of said notches are larger than acute angles.

8. A fusible element for an electric fuse which has a reduced-width section and which has a second reducedwidth section, said reduced-width sections being displaced axially of said fusible element and coacting with each other to define a weak spot that has a short effective length.

9. A fusible element as claimed in claim 8 wherein the first said reduced-width section provides a rnuch smaller electrical resistance and a much larger heat-conducting capacity per unit of length than said weak spot can provide, and wherein said second reduced-width section provides a much smaller electrical resistance and a much larger heat-conducting capacity per unit of length than said weak spo can provide.

10. A fusible element as claimed in claim 8 wherein the first said reduced-width section has one edge thereof adjacent the center line of said fusible element and extends toward the opposite edge of said fusible element, and wherein said second reduced-width section has one edge thereof adjacent the center line of said fusible element and extends toward the opposite edge of said fusible element.

11. A fusible element for an electric fuse which has an open area therein and which has a second open area therein, the first said open area having a part of an edge thereof substantially in register with a part of an edge of said second open area to define a weak spot, the first said open area having substantially all of the length of said edge thereof displaced axially in one direction from said part of said first open area, said second open area having substantially all of the length of said edge thereof displaced axially in the opposite direction from said part of said second open area, whereby said weak spot has a short effective length.

12. A fusible element as claimed in claim 11 wherein said first said open area is a notch and said second area is an opening.

13. A fusible element as claimed in claim 11 wherein said first said open area is an opening and said second area is an opening.

14. A fusible element as claimed in claim 11 wherein 7 8 said first and said second open areas have corners 2,502,747 4/ 1950 Popp 200--135 with large angles. 2,833,891 5/ 1958 Jacobs 200131 2,982,835 5/1961 Kozacka 200-120 X References Cited H. B. GILSON, Assistant Examiner.

1,266,479 5/1918 Jensen 200-135 X 1,441,550 1/1923 Weston 2o0 135 2,644,060 6/1953 Von Hoorn 200-435 2,682,587 6/1954 Burt et al. 200-131 10 s3 7 227,22s 55;, 

